Apparatus for production of polyethylene

ABSTRACT

Polyethylene is produced by polymerization of ethylene along or with comonomers or telogens (modifiers) in an elongated tubular reactor having an inlet and outlet and at least one reaction zone and one cooling zone in the presence of free radical or free oxygen yielding initiator at elevated temperatures and pressures by passing the reaction mixture through each of the reaction zones of the tubular reactor having internal diameters between about 0.5 and 3 inches at bulk fluid velocities sufficient so that the Flow Number in each reaction zone is greater than 3.3 ft2./sec. Flow Number is defined as the bulk fluid velocity in ft./sec. times diameter in feet. With Flow Numbers in excess of 3.3 ft2./sec. in the reaction zones, it has been found that the effective reaction volume in the reaction zone has been increased and, accordingly, a more efficient process for producing a high quality polyethylene.

United States Patent lnventors Charles D. Beals;

George I. Fitzpatrick; Kim L. OHara, all of Baton Rouge, La.

May 15, 1970 Dec. 21, 1971 Esso Research and Engineering Company App].No. Filed Patented Assignee APPARATUS FOR PRODUCTION OF POLYETHYLENE 9Claims, 4 Drawing Figs.

References Cited UNlTED STATES PATENTS 6/1942 Gage 9/ l 94 3 Frey sinszone zone PREHEATER 2" 3 2,856,395 10/1958 Richard, Jr. et a1 260/95 X3,003,853 10/1961 Mecorney et al. 23/285 X 3,103,942 9/1963 Shays137/561 X Primary Examiner-James H. Tayman, Jr.

Attorneys-Thomas B. McCulloch, Melvin F. Fincke, John S.

Schneider, Sylvester W. Brock, Jr., Kurt S. Myers and Timothy L. BurgessABSTRACT: Polyethylene is produced by polymerization of ethylene alongor with comonomers or telogens (modifiers) in an elongated tubularreactor having an inlet and outlet and at least one reaction zone andone cooling zone in the presence of free radical or free oxygen yieldinginitiator at elevated temperatures and pressures by passing the reactionmixture through each of the reaction zones of the tubular reactor havinginternal diameters between about 0.5 and 3 inches at bulk fluidvelocities sufficient so that the Flow Number in each reaction zone isgreater than 3.3 ft Jsec. Flow Number is defined as the bulk fluidvelocity in ft./sec. times diameter in feet. With Flow Numbers in excessof 3.3 ft jsec. in the reaction zones, it has been found that theeffective reaction volume in the reaction zone has been increased and,accordingly, a more efficient process for producing a high qualitypolyethylene.

INITIATOR PATENIEB W221 Bil ATTORNEY.

APPARATUS FOR PRODUCTION OF POLYETHYLENE BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention is directed to thepolymerization of ethylene alone or with comonomers or telogens(modifiers) at elevated temperatures and pressures in an elongatedtubular reactor. More particularly, the invention is directed to theproduction of solid polyethylene under conditions wherein the bulk fluidvelocity is sufficiently high in the reaction zones of the tubularreactor so that the Flow Number in each reaction zone is greater than3.3 ftF/sec. In its more specific aspects, the invention involves theproduction of high quality polyethylene under conditions wherein thebulk fluid velocity is sufficiently high so that the Flow Number in eachreaction zone in the tubular reactor having internal diameter betweenabout 0.5 and 3 inches is greater than 3.3 ftF/sec. and the effectivereaction volume is increased to produce high quality polyethylene whilecontrolling the pressure drop in the tubular reactor having at least tworeaction zones so as not to exceed 6,000 and, preferably, 3,000 p.s.i.at operating pressures between 25,000 p.s.i. and 50,000 p.s.i. at theinlet of the tubular reactor as calculated or measured between the inletof the first reaction zone and the end of the last of the reactionzones.

2. The Prior Art The polymerization of ethylene to solid polyethylene inan elongated tubular reactor at elevated temperatures and pressures inthe presence of a free radical or free oxygen yielding initiator isknown. Heretofore, however, the use of high bulk fluid velocities hasbeen carefully avoided due to the increased lengths of reaction zonesthought to be necessary to accommodate the increased velocities inconventional tubular reactors. It has been understood heretofore that toprovide a constant temperature rise per unit length of reaction zone inthe tubular reactor, a doubling of the bulk fluid velocity would doublethe length of reaction zone. Hence, increasing the bulk fluid velocityin the reaction zone would necessarily increase the length thereofresulting in a much greater pressure drop occurring in the tubularreactor. ln tubular reactors having one long or more than one shorterreaction zone, large pressure drops have detrimental effects on productoptical property and uniformity of other physical properties since thepolymer produced in the second or later portion of the reaction zone orzones are produced under lower pressures. The base film product isproduced at the highest pressure. Since for practical purposes thereaction zone must be of a finite length and increasing the length ofthe reaction zone increase the pressure drop, the bulk fluid velocitieshave not exceeded about 36fL/sec. in a l-inch pipe; i.e., a Flow Numberof about 3.0.

SUMMARY OF THE INVENTION The present invention may be briefly describedas a process and apparatus for producing polyethylene in a tubularreactor having at least one reaction zone at elevated temperature andpressure conditions at bulk fluid velocities sufficient so that the FlowNumber in each reaction zone in the tubular reactor having internaldiameters between 0.5 and 3 inches is greater than 3.3 ftF/sec.Significant to the present invention is the finding that at a FlowNumber greater than 3.3 ttP/sec. the reaction zones having internaldiameters between 0.5 and 3 inches operate at the lower end of theturbulent flow regime. This turbulent flow regime is characterized by afully developed turbulent flow core, a buffer region and a laminar flowsublayer close to the tube wall. The flow regime was found by obtainingaccurate pressure drop measurements which heretofore have not beenobtainable with polyethylene polymerization reactions. Anothersignificant finding to the present invention is that the major portionof the reaction occurs in the fully developed turbulent flow core of theturbulent flow regime. Still another significant aspect of the presentinvention is that pressure drop may be controlled by use of side streamsand/or use of larger diameter cooling tubes making up the tubularreactor. More particularly, therefore, the present invention is directedto a process and apparatus for producing polyethylene in a tubularreactor having at least one reaction zone at elevated temperature andpressure conditions at bulk fluid velocities sufficient so that the FlowNumber in each reaction zone in the tubular reactor having internaldiameters between 0.5 and 3 inches-is greater than 3.3 ft./sec. so thatthe effective reaction volume is increased while controlling thepressure drop in the tubular reactor, wherein there are more than onereaction zone, between the inlet of the first reaction zone and the endof the last of the re action zones so as not to exceed about 6,000p.s.i. when operating at pressures between 25,000 and 50,000 p.s.i. atthe inletof the first reaction zone.

VARIABLES or THE INVENTION The present invention is not limited to anyspecific tubular reactor design, catalyst or initiator system ortemperature or pressure conditions. Accordingly, these variables, whileimportant to any specific process or apparatus, are set forth generallyas background for the present invention.

The tubular reactor may be an elongated jacketed tube or pipe, usuallyin sections, of suitable strength and having an inside diarneter betweenabout 0.5 to about 3 inches or more. The tubular reactor usually has alength to diameter ratio above about to 1, and, preferably, having aratio from 500 to l to about 25,000 to l.

The tubular reactor is operated at pressures from about 1,000 to about4,000 atmospheres. Pressures higher than 4,000 atmospheres may be used,but a preferred range is about 2,000 to about 3,500 atmospheres.

The temperature employed are largely dependent on the specific catalystor initiator system used. Temperatures may range from about 300 to about650 F. or higher. The catalyst or initiator is a free radical initiatorwhich may include oxygen, peroxidic compounds, such as hydrogenperoxide, decanoyl peroxide, diethyl peroxide, di-t-butyl peroxide,butyryl peroxide, t-butyl-peroctate, di-t-butyl peracetate, lauroylperoxide, benzoyl peroxide, t-butyl peracetate, alkyl hydroperoxides,azo compounds, such as azobisisobutyronitrile, alkali metal persulfates,perborates, and percarbonates, and oximes, such as acetoxime to mentiononly a few. A single initiator or a mixture of initiators may be used.

The feedstock employed in the present invention may be ethylene orpredominately ethylene together with a telogen (modifier) or comonomer.Known telogens or modifiers, as the term is used herein, are illustratedby the saturated aliphatic aldehydes, such as formaldehyde, acetaldehydeand the like, the saturate aliphatic ketones, such as acetone, diethylketone, diamyl ketone, and the like, the saturated aliphatic alcohols,such as methanol, ethanol propanol, and the like, parafiins orcycloparaffins such as pentane, hexane, cyclohexane, and the like,aromatic compounds such as toluene, diethylbenzene, xylene, and thelike, and other compounds which act as chain terrninating agents such ascarbon tetrachloride, chloroform, etc. The process of the presentinvention may also be used to produce copolymers of ethylene with one ormore polymerizable ethylenically unsaturated monomers having a present.Polymerizable ethylenically unsaturated monomers havinga CHz C group andwhich undergo addition polymerization are, for example, alphamonoolefins, such as propylene, butenes, pentenes, etc.. the acrylic,haloacrylic and methacryclic acids, esters, nitriles and amides, such asacrylic acid, chloroacrylic acid, methyacrylic acid, cyclohexylmethycrylate, methyl acrylate, acrylonitrile, acrylamide; the vinyl andvinyl-idene halides; the N-vinyl amides; the vinyl carboxylates, such asvinyl acetate; the N-vinyl aryls, such as styrene; the vinyl ethers,ketonen or other compounds, such as vinyl pyridine, and the like.Comonorners and telogens or modifiers are used to modify the propertiesof the ethylene polymer produced. Accordingly, the tenn polyethylene, asused herein, is meant to include the so modified ethylene polymers aswell as homopolyethylene.

BRIEF DESCRIPTION OF DRAWINGS The present invention will be furtherillustrated by reference to the drawings in which:

FIG. I is a flow diagram of a constant diameter tubular reactor havingtwo reaction zones for the polymerization of ethylene;

FIG. 2 is an illustration in cross section of the flow regime through areaction zone of a tubular reactor according to the present invention;

FIG. 3 is a flow diagram of a tubular reactor having two reaction zonesand a cooling zone of increased diameter; and

FIG. 41 is an illustration of a tubular reactor having three reactionzones according to this invention and provided with side streams anddifferent diameter tubes to provide high conversion, increased effectivereaction volume, i.e., reaction zone boundary layer control and pressuredrop control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention may beillustrated by referring to FIG. 1 of the drawings which illustrate atubular reactor of constant diameter having two reaction zones,Referring to FIG. 1, numeral designates a feedline for introducingethylene from a source (not shown) into a primary compressor ii. Theethylene feed which may also include modifiers or comonomers is then fedthrough line 12 to secondary compressor 13. The secondary compressor 13,the feed gases are compressed for introduction into the tubular reactor14 at temperature and pressure conditions such that the gases are atsupercritical conditions. Modifiers or cornonomers may be introducedinto line 12 through line 15 and, in addition, recycle gases may beintroduced into line 12 through line 16 before secondary compressor 13.The compressors illustrated in FIG. 1 may be a single compressor or twoor more in series or parallel. The side streams or recycle streams mayuse separate compressors or lines from the primary or secondarycompressors.

The feed stream, after being introduced into tubular reactor 14, comesinto contact with an initiator introduced through line 17 whereby areaction occurs in reaction zone 13. The tubular reactor 14 is jacketedalong essentially its entire length with heat exchange media in thejacket to provide cooling for the highly exothermic reaction. After amaximum temperature is reached in the reaction zone, the reactionmixture cools in cooling zone 18C. Additional initiator may be addedthrough line 19 to provide a second reaction zone 20 wherein again amaximum temperature is reached followed by cooling in cooling zone 20C.The tubular reactor M is illustrated having two reaction zones; however,if additional reaction zones are desired, additional initiator may beadded by lines, such as line 21. At the end 22 of tubular reactor 14,the reaction mass is discharged through a pressure let down valve 23into line 24 which leads to a high-pressure separator 25.

In the high-pressure separator 25, unreacted ethylene, modifiers and/orcomonomers are separated and recycled through line 16. The gases areappropriately cooled and wax separated (not shown) before recycled toline 12. The polymer is removed from the bottom of the high-pressureseparated 25 through line 26 wherein it may pass through a second letdown valve 27 and through line 28 into a low-pressure separator 29.

In low-pressure separator 29, gases are removed through line Fill. Thedesired polyethylene is removed through line Ell to be finished in aconventional manner.

In arriving at the present invention, a significant aspect was theability to measure the pressure drop in the tubular reactor M. This wasaccomplished as illustrated in FIG. 1 by having a positive displacementpump 60 pump against the head at the end 22 of the tubular reactor 14.li-Iexane was pumped in very small amount, less than 1 gallon per hour,through line 42 having a check valve &3 therein. instead of hexane, anyinert and/or compatible fluid with the polymer produced may be used. Thepositive displacement pump 410 having a pressure rating of about 60,000p.s.i. had a pressure gauge (not shown) thereon. As the hexane waspumped through line d2, the pressure therein was the same as thepressure at the end .22 of the tubular reactor 14) and at any point intime was shown on the pressure gauge as the head against which the pumpM was pumping. By this technique, pressure measurements were obtainedwhich heretofore were unobtainable, because of difficulties such asplugg'ng caused by the presence of the polymer in attempting to useconventional pressure measuring devices, such as a gauge or strain gaugedevices. As the let down valve 22 opened under the normal pulsing orbumping cycle, hexane continued to flow in line 42 which preventsplugging therein and provides accurate pressure measurements.

The following experimental data was obtained in a constant diametertubular reactor such as illustrated in FIG. 1. The results of theexperimental runs are set forth in table I below. The experimental runswere at differing bulk flow velocities and a startling result was foundthat higher bulk fluid velocities than previously employed weredesirable to produce uniform, improved product. This was particularly sosince the additional reaction volume required at higher bulk fluidvelocities, normally considered in terms of additional length of tubularreactor, was obtained in reactor lengths essentially that used at muchlower bulk fluid velocities. To express the present invention, acritical bulk flow velocity could not be defined since it would bemeaningful only for a specific diameter tube or pipe. In other words,the numerical value of the bulk fluid velocities which are necessary toobtain the results of the present invention would be different for eachdiameter pipe used in a tubular reactor. Accordingly, the term FlowNumber is used herein to express the nature of the flow regime in atubular reactor according to the present invention. Flow Number isdefined as the bulk fluid velocity in ft./sec. times the diameter infeet. In the following table, a comparison is given among runs atdifferent bull: flow velocities and at approximately the sameconversions, in a tubular reactor of l inch inside diameter.

ln the above table, V is a bulk fluid velocity in ft./sec. The FN isFlow Number and MI is melt index. The pressure drop (AP) is thatobtained by calculation from the measured pressure drop between theinlet and outlet of the tubular reactor for that portion of the tubularreactor between the inlet to the reactor and the end of the lastreaction zone. It is this AP which is meaningful to produce qualitysince no further reaction occurs and, accordingly, the pressure dropoccuring in the last cooling zone is simply a matter of choice.

It can be seen from table I that an improved film product is made at thehigher Flow Numbers in that the haze is lower and gloss higher. Thepressure drop is shown to have an effect on product quality since at thelarger pressure drops, the pressure in the second reaction zone is muchlower than in the first reaction zone and, accordingly, the polymerproduced therein is different.

It was found in analyzing the data from the experimental runs that theflow regime of the reaction mixture in the reaction zones of the tubularreactor was no longer in the higher portion of laminar flow, butpartially turbulent flow rem'me. Hence, in calculating a friction factor(I) using the fundamental equation relating to flow of fluids in a pipe,the friction factor was proportional to the Reynolds number to the minusone-fourth power. Significantly, this detemtination led to the presentinvention, wherein the bulk fluid velocities should be increased in thepolymerization of polyethylene sufficiently to provide in the reactionzones of tubular reactors having internal diameters between 0.5 and 3inches, a Flow number greater than 3.3 ftF/sec. The nature of the flowregime in the reaction zones of the tubular reactor is illustrated inFIG. 2. The partially turbulent flow regime in reaction zone 18, forexample, comprises a fully turbulent flow core 45, a buffer region (notshown) and a laminar flow sublayer 46. The line 47 represents thevelocity profile and indicates that the velocity in the turbulent flowcore 45 is greater than in the laminar flow sublayer 46. Moresignificantly, however, now knowing the nature of the flow regime atthese bulk fluid velocities, it was concluded that even greater bulkfluid velocities for maximum turbulent volume were desired. Tosubstantiate this conclusion, the effective volume of the turbulent flowcore 45 and the percentage of the thickness of the laminar flow boundarylayer 46 were calculated. The results are shown in table II using thedata obtained in the same runs set forth in table I.

TABLE II Effective Volume (15 Total Volume) Laminar Thickness concludedfrom the data were the following: that the major portion of the reactionoccurs in the turbulent flow core 45, that the boundary layer thicknessor percentage of laminar thickness should be minimized to increaseeffective reaction volume, that a more uniform product is produced atincreased effective volumes, that the bulk fluid velocities should beincreased over any presently used and sufficient to produce a FlowNumber greater than 3.3 ft./sec., preferably greater than 3.5 ft./sec.,and suitably in the range of 4 to 10 ftF/sec. for reaction zones intubular reactors having diameters between 0.5 and 3 inches, and thatpressure drop should be controlled in tubular reactors having more thanone reaction zone to less than about 6,000 p.s.i. and, preferably, lessthan 4,000 p.s.i. at inlet pressures between 25,000 and 50,000 p.s.i.since large pressure drops between reaction zones affect product qualityand uniformity.

To illustrate the operability at very high Flow Numbers, an experimentalrun was made in a one inch diameter tubular reactor. Only one reactionzone was used due to limited heat transfer surface in the reactor. Theresults are set forth in table II]. Pressure drop at this flow ratewould have prohibited product of high quality film product if a secondreaction zone were added.

TABLE III v FN AP Ml Haze Gloss zone 2 l It may be concluded from theforegoing data in tables 1 and ill! that a Flow Number geater than 3.3ftF/sec. produces an improved product, but that pressure drop must becontrolled if a second reaction zone is used to obtain improved qualityfilm product. Accordingly, one aspect of the present invention is tocontrol the pressure drop in tubular reactors for producing polyethyleneby using either side stream injection or larger diameter cooling tubes,or both. This aspect of the present invention is illustrated withreference to FIG. 3, wherein ethylene either alone or together withmodifiers and/or comonomers is introduced by line 50 into secondarycompressor 51. The gases are then introduced by line 52 into a tubularreactor 53 having initial tubes of a particular diameter. Initiator isintroduced through line 54 into the initial portion of the tubularreactor wherein reaction occurs in zone 55. At that point in the tubularreactor 53, where the temperature reaches a maximum, and cooling begins,tubes of a larger diameter are used to provide a cooling zone 56. inaddition, if desired, cooling media may be introduced through sidestream 57. The cooling media may be ethylene, modifier, comonomer, aninert diluent or combinations thereof. After the reaction mixture passesthrough the cooling zone 56 of the tubular reactor 53, the mixture isintroduced into a second reaction zone 58 wherein additional initiatoris introduced by line 59. if not already introduced in line 57. Thereaction mixture is then passed through the tubular reactor 53 andthrough let down valve 60. The reaction mixture is then passed throughthe tubular reactor 53 and through let down valve 60. The reactionmixture is then separated in a separator 61 in a conventional mannerwith the gases being recycled through line 62 and the polymer productpassing through line 63. According to the present invention, coolingtubes of larger diameter are used between reaction zones 55 and 58 toprovide a lower pressure drop between reaction zones because of theshorter length of tube required. Furthermore, a side stream 57 may beprovided to aid in cooling which will also reduced the length of thecooling zone required and the pressure drop between re action zones. Byeither or both of these means, the bulk fluid velocities may beincreased over presently used velocities suffciently to produce FlowNumbers greater than 3.3 ft./sec., preferably 3.5 fl./sec. or more, ineach reaction zone having an internal diameter between 0.5 and 3 inchesso as to increase the effective reaction volume or the volume of theturbulent flow core while producing a higher quality product Bycontrolling the pressure drop to less than 6,000 p.s.i., preferably lessthan 4,000 p.s.i., between the inlet of the tubular reactor having apressure between 25,000 and 50,000 p.s.i. and the end of the lastreaction zone in the tubular reactor, high quality film product may beproduced.

To illustrate the present invention in all of its aspects, a tubularreactor for the production of polyethylene is illustrated in FIG. 4. Thefeed gases are introduced by line 70 to a preheater 71 to be heatedbefore introduction into the jacketed tubular reactor 72. The tubularreactor 72 has six zones including three reaction zones and threecooling zones. Initiator is introduced into reaction zones ll, 3, and 5,while side streams for cooling and reducing pressure drop are introducedinto cooling zones 2 and 4. The following table lV sets forth theinternal diameter of the tubes in each zone, the bulk fluid velocitiesin each zone and the Flow Number in each reaction zone for a particulardesign. In any particular design, the first reaction zone may have aninternal diameter between 0.5 and 2.5 inches.

TABLE IV lntemal Diameter (lnches) Bulk Fluid Velocity (it/sec.) FN

Zone 1 I 52.5 52.5

Zone 3 L25 Zone 4 L50 39.2 Zone L25 79.0 8.23 Zone 6 1.50 55.0

The pressure drop in the tubular reactor 72 having the: foregoingdimensions would be less than 2,500 p.s.i. between the inlet zone to 1and the end of zone 5 at inlet pressures in excess of 35,000 p.s.i. Thetwo different diameters for the first cooling zone 2 illustrates thatmaintaining the same internal diameter tubes for a certain length toprovide cooling before increasing the internal diameter is within thepresent invention and is a matter of design. For example, all of coolingzone 2 may be 1.25 inches internal diameter. However, it is desirable tointroduce the side stream to the larger diameter tube. The length of thecooling zone in the larger diameter tube may be very short (less than 50ft.) before another reaction is initiated by introduction of theinitiator. While various design changes may be made, according to thepresent invention, the important feature is that the bulk fluidvelocities in each reaction zone are sufficient to provide a Flow Numbergreater than 3.3 ftF/sec. Further, it is desirable to have larger FlowNumbers in each successive reaction zone so as to provide greatereffective reaction volumes to ofi'set the effect of having polymer inthe reaction mass in the subsequent reaction zone. With the tubularreactor 72, illustrated with the foregoing dimensions, the effectivereaction volume is 64 percent in zone 1, 72 percent in zone 3, and 84percent in zone 5. Since essentially no reaction is occurring in thecooling zones 2, 4, and 6, the flow regime is not critical to the kindof product produced and, ac cordingiy, much lower bulk fluid velocitiesin the larger diameter cooling tubes may be used. The larger diametertubes provides additional heat transfer surface as well as reducesoverall pressure drop.

ln summary, the present invention is directed to a process for producingpolyethylene in reaction zones having internal diameters between 0.5 and3 inches at bulk fluid velocities sufficient so that the Flow Number isgreater than 3.3 ftF/sec. in each reaction zone. Preferably, tubularreactors having at least two reaction zones or more are used and theFlow Number in each successive reaction zone will be equal to or,preferably, higher than the reaction zone it follows. Further, intubular reactors having at least two reaction zones, the pressure dropbetween the inlet of the tubular reactor and the end of the lastreaction zone is controlled to less than 6,000 psi, preferably 4,000psi. when the inlet pressures are between 25,000 and 50,000 psi. Thecontrol of pressure drop is accomplished by side stream injection and/orlarger diameter tubes in the cooling zones.

The nature and object of the present invention having been completelydescribed and illustrated and the best mode contemplated set forth.

What we wish to claim as new and useful and secure by Letters Patent is:

l. A tubular reactor for producing polyethylene under high pressureconditions which comprises:

at least three tube means having different internal diameters within therange between 0.5 and 3.0 inches interconnected to provide an inlet andan outlet,

said first tube means having an internal diameter between 0.5 and 2.5inches,

said second tube means having a larger internal diameter than said firsttube means,

said third tube means having an internal diameter at least that of saidfirst tube means and less than said second tube means,

the: interconnection between each of said tube means serving,respectively, as an outlet for the first tube means and an inlet for thesecond tube means and an outlet for the second tube means and an inletfor the third tube means, and

said first and third tube means having an inlet means, the inlet meansfor the first tube means being near its inlet, and the inlet means forthe third tube means being near its inlet. 2. A tubular reactor inaccordance with claim l! wherein the second tube means has a side streaminlet means between its inlet and its outlet.

3. A tubular reactor in accordance with claim 1 wherein the inlet meansin said first and third tube means are connected to means for supplyinginitiator.

4. A tubular reactor according to claim 2 wherein the side stream inletmeans in said second tube means is connected to means for supplyingcooling media.

5. A tubular reactor having a length to diameter ratio at least -to -lfor producing polyethylene under high-pressure conditions whichcomprises;

at least five tube means having different internal diameters within therange between 0.5 and 3.0 inches interconnected to provide an inlet andan outlet,

said first tube means having an internal diameter between 0.5 and 2.5inches,

said second tube means having a larger internal diameter than said firsttube means,

said third tube means having a larger internal diameter than said secondtube means,

said fourth tube means having an internal diameter at least that of saidsecond tube means and less than said third tube means, and

said fifth tube means having a larger internal diameter than said fourthtube means,

the interconnection between each of said tube means serving,respectively, as an outlet for the first tube means and an inlet for thesecond tube means and an outlet for the second tube means and an inletfor the third tube means, an outlet for the third tube means, an outletfor the fourth tube means and an inlet for the fifth tube means, saidfirst, second and fourth tube means having inlet means, the inlet meansfor the first tube means being near its inlet, the inlet means for thesecond tube means being near its inlet, and the inlet means for thefourth tube means being near its inlet.

6. A tubular reactor in accordance with claim 5 wherein the second andfourth tube means each has a side stream inlet means between theirinlets and outlets, respectively.

7. A tubular reactor in accordance with claim 5 wherein the inlet meansin said first, third and fifth tube means are connected to means forsupplying initiator.

8. A tubular reactor in accordance with claim 5 wherein the inlet meansfor the second and fourth means are connected to means for supplyingcooling media.

9. A tubular reactor according to claim 5 wherein the diameter of saidtube means are as follows:

First 1 inch Second l.25 inch Third 1.50 inch Fourth 1.25 inch Fifth1.50 inch.

2. A tubular reactor in accordance with claim 1 wherein the second tubemeans has a side stream inlet means between its inlet and its outlet. 3.A tubular reactor in accordance with claim 1 wherein the inlet means insaid first and third tube means are connected to means for supplyinginitiator.
 4. A tubular reactor according to claim 2 wherein the sidestream inlet means in said second tube means is connected to means forsupplying cooling media.
 5. A tubular reactor having a length todiameter ratio at least 100-to -1 for producing polyethylene underhigh-pressure conditions which comprises; at least five tube meanshaving different internal diameters within the range between 0.5 and 3.0inches interconnected to provide an inlet and an outlet, said first tubemeans having an internal diameter between 0.5 and 2.5 inches, saidsecond tube means having a larger internal diameter than said first tubemeans, said third tube means having a larger internal diameter than saidsecond tube means, said fourth tube means having an internal diameter atleast that of said second tube means and less than said third tubemeans, and said fifth tube means having a larger internal diameter thansaid fourth tube means, the interconnection between each of said tubemeans serving, respectively, as an outlet for the first tube means andan inlet for the second tube means and an outlet for the second tubemeans and an inlet for the third tube means, an outlet for the thirdtube means, an outlet for the fourth tube means and an inlet for thefifth tube means, said first, second and fourth tube means having inletmeans, the inlet means for the first tube means being near its inlet,the inlet means for the second tube means being near its inlet, and theinlet means for the fourth tube means being near its inlet.
 6. A tubularreactor in accordance with claim 5 wherein the second and fourth tubemeans each has a side stream inlet means between their inlets andoutlets, respectively.
 7. A tubular reactor in accordance with claim 5wherein the inlet means in said first, third and fifth tube means areconnected to means for supplying initiator.
 8. A tubular reactor inaccordance with claim 5 wherein the inlet means for the second andfourth means are connected to means for supplying cooling media.
 9. Atubular reactor according to claim 5 wherein the diameter of said tubemeans are as follows: First 1 inch Second 1.25 inch Third 1.50 inchFourth 1.25 inch Fifth 1.50 inch.